How humans adapt to negative energy balance is critical for understanding the physiology of energy storage and more broadly how mammals maintain energy stores during food scarcity. Also, understanding negative energy balance is crucial for understanding and treating obesity, which is epidemic in developed countries. During negative energy balance, fat stores diminish and energy expenditure declines. Previous studies have documented that during energy restriction, only small changes in basal metabolic rate and the thermic effect of food occur, implying that activity thermogenesis (AT) may contribute substantively to energy modulation during negative energy balance. In this proposal we will examine AT during negative energy balance in lean and obese human subjects. In Specific Aim 1 we will address the hypothesis that AT decreases during energy restriction in lean individuals. Here, we will not only ascertain how non-exercise activity thermogenesis (NEAT) changes during 1000 kcal/day negative energy balance but also tease apart how the components of AT change. Specifically, we will study the effects of negative energy balance on, sitting, standing, walking, changing posture and fidgeting thermogenesis. In Specific Aim 2 we will address the hypothesis that obesity is associated with exaggerated decline in NEAT during underfeeding. Here, we will energy restrict obese and lean subjects and measure NEAT and its components before and after underfeeding. Also, to ascertain whether differences between the obese and lean subjects relate to excess adiposity alone or to the susceptibility of an individual to obesity, we will similarly underfeed a group of post-obese subjects. Our third specific aim is incredibly exciting. Here, we want to determine whether an imposed 200 kcal/day of walking in addition to energy restriction, attenuates the decline in AT with underfeeding in lean, obese and post-obese subjects. We want to know this, in part, from a mechanistic perspective in order to gain insight into how volitional AT is impacted by imposed walking. Also, this will help us to better understand how to treat our obese patients and to define the energetic benefit of combining a walking program with energy restriction. In conclusion, this program will allow us to define for the first time how AT, NEAT and its components change during negative energy balance in lean individuals. Next, these studies will provide us with enormous insight regarding how obese patients adapt energetically during negative energy balance. Finally, we will gain fundamental information regarding the metabolic implications of combining food restriction with a walking program compatible to that advocated by statutory agencies. These studies will lead to improved understanding of the energetic adaptation that occurs during negative energy balance and how best to treat patients with obesity.